School me on crossover slopes

[jtvang125]

I have a basic understanding of what they are but can someone explain what slope I should use for various applications?

I have a car headunit that has fully adjustable crossover frequencies and slope for front, rear and subs.

 

[Howard]

Can of worms right here.

Why don't you start by explaining what you want to do?

 

[bobdole369]

In a nutshell - the basic premise is that you have a series of driver sets. Lets say you have a 3 way system:

Subwoofer in the back
Lo/Midrange in the doors
High in the dash.

Your subwoofer should be getting 60hz and down (tho some swear by 100hz and down).
Low/Mid in the doors should get roughly from whatever value your subs roll off - up to about 800hz-2khz - depends on your system, drivers, soundstange etc.
High in the dash gets from that value up beyond 20khz.

Now your crossover slope is the rate at which a crossover network rolls off. As we cannot economically create a brickwall effect for crossovers (that is a component that by which once a certain frequency is reached anything above it in a low pass or below it in a high pass is immediately stopped, nor would we actually want to (it would sound like garbage). Your crossover slopes "roll-off". It is actually an electrical property of inductors/capacitors/resistors, but essentially when talking about crossover slopes what you refer to is the rate at which the network rolls off.

So lets say you have a sine wave at 40hz. Your subwoofer is banging it out full blast. Your crossover is at 60hz and you have a 12db/octave rolloff.
Now take your sine wave and move it up to 60hz. Now your low speakers start making noise and your subwoofer starts making less noise. Specifically how much is what the crossover slope is referring to.

At the "crossover point" in a 12db network, the signal presented to the driver will be 6db down from "0db" which is the reference point or full blast. Also the signal presented to the "low" driver will be 6 db down. This produces a slight dip of about 3db at the crossover point, most installers will agree that you need to overlap your crossover points in order to avoid this dip.

Now take your sine wave at 60hz again, now you have a 6db/octave rolloff. At the crossover point you will be 3db down on the sub, and 3 db down on the low. If you are moving up in frequency the rate at which the xover network reduces the signal to the sub and increases the signal to the low driver is that crossover slope.

an octave is a doubling in frequncy. So 60hz to 120hz is one octave. So if 60hz is your crossover point and you have a 6db/octave crossover - it starts reducing signal to the sub, roughly 45hz. At 60hz it is 3db down, and at 90hz it is 6db down. At 180hz it is 12db down.

 

[Tiamat]

Additionally... the steeper the slope, the more complex the circuit will be to arrive at such a filter. Also, the phase angle of the signal throughout the crossover range becomes more severe as the slope of the filter becomes steeper. On the flip side, the more gentle the slope, the simplier the circuit for such a filter and the more uniform your phase angle vs. frequency will be.

Generally, the gentler slopes force the speakers to play outside their comfortable reproduction range. Depending on the make/model, some speakers can handle this better than others without distortion. If your model of speaker has terrible response outside its recommended band, then you might want to consider steeper sloped crossover filters. However, as the phase angle becomes more severe, the more current will be demanded from your amplifier and if your amp cannot handle the current, it will overheat. The effect on the phase angle depends on the type of filter design you choose among other things...

I'm not a EE and I do not have a very complete understanding of circuit design, but when it comes to crossovers, it is rarely something you can just "pick and it works". There are many design considerations to think about.

 

[scott916]

Ideally, use the highest slope crossover available, on head units it's likely 12 or 18dB/octave. If you can, the best way to do it is to look at the response curves of your speakers. Cut the crossover so that it most closely cuts the frequency before the rolloff of the driver. If you need me to explain further, I'll grab a response curve graph and show you what I mean. Proper crossover settings really make a huge difference in how speakers sound.